From 286f116d15b8535a9c085f64bfd95ab85d9a3232 Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Thu, 5 Jan 2023 22:22:25 -0800
Subject: [PATCH] experiment with aliasing

---
 main.c  |  3 +--
 sh2.lds |  2 +-
 type.h  | 11 +++++++++--
 vdp2.h  |  9 +++++++--
 4 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/main.c b/main.c
index 1187366..66605f1 100644
--- a/main.c
+++ b/main.c
@@ -6,8 +6,7 @@ void start(void) {
 
   vdp2.reg.BGON = 0;
 
-  vdp2.reg.BKTAU = BKTAU__BKCLMD_SINGLE_COLOR | 0;
-  vdp2.reg.BKTAL = 0x4000;
+  vdp2.reg.BKTA = REG32_VAL(BKTAU__BKCLMD_SINGLE_COLOR, 0x4000);
 
   *((reg16 *)&vdp2.vram[0x8000]) = 31; // red
 
diff --git a/sh2.lds b/sh2.lds
index 400cd18..553599a 100644
--- a/sh2.lds
+++ b/sh2.lds
@@ -7,7 +7,7 @@ MEMORY
 }
 SECTIONS
 {
-  . = 0x06000000;
+  . = 0x06003000;
 
   .text ALIGN(2) : SUBALIGN(2)
   {
diff --git a/type.h b/type.h
index 002e7c6..72ba498 100644
--- a/type.h
+++ b/type.h
@@ -1,5 +1,12 @@
+#define static_assert _Static_assert
+#define offsetof __builtin_offsetof
+
 typedef volatile unsigned char reg8;
 typedef volatile unsigned short reg16;
 typedef volatile unsigned long reg32;
-#define static_assert _Static_assert
-#define offsetof __builtin_offsetof
+
+static_assert((sizeof (reg8)) == 1);
+static_assert((sizeof (reg16)) == 2);
+static_assert((sizeof (reg32)) == 4);
+
+#define REG32_VAL(U, L) (((U) << 16) | (L))
diff --git a/vdp2.h b/vdp2.h
index 8cd898f..a8d6176 100644
--- a/vdp2.h
+++ b/vdp2.h
@@ -94,8 +94,13 @@ typedef struct vdp2_reg {
   reg16 LSTA1L; /* LINE SCROLL TABLE ADDRESS (NBG1) */
   reg16 LCTAU;  /* LINE COLOR SCREEN TABLE ADDRESS */
   reg16 LCTAL;  /* LINE COLOR SCREEN TABLE ADDRESS */
-  reg16 BKTAU;  /* BACK SCREEN TABLE ADDRESS */
-  reg16 BKTAL;  /* BACK SCREEN TABLE ADDRESS */
+  union {
+    struct {
+      reg16 BKTAU;  /* BACK SCREEN TABLE ADDRESS */
+      reg16 BKTAL;  /* BACK SCREEN TABLE ADDRESS */
+    };
+    reg32 BKTA;
+  };
   reg16 RPMD;   /* ROTATION PARAMETER MODE */
   reg16 RPRCTL; /* ROTATION PARAMETER READ CONTROL */
   reg16 KTCTL;  /* COEFFICIENT TABLE CONTROL */